Buffer Overflow in Hanami with Hmac Signatures

A buffer overflow occurs when an application writes more data to a fixed-length buffer than it can hold, corrupting adjacent memory. In Hanami, using Hmac Signatures for request authentication can inadvertently expose this class of vulnerability if input handling is not strictly bounded before the signature is computed or verified.

When a Hanami endpoint accepts parameters that influence the size of an in-memory buffer—such as a payload length field or a user-controlled header used to allocate a string—and those parameters are included in the Hmac computation without validation, an attacker can supply an oversized or malformed value. This can lead to reading or writing beyond the intended memory region, potentially altering control flow or causing crashes.

Consider a scenario where a request includes a JSON body with a data field and an X-Message-Length header. If the application uses the header to allocate a fixed-size buffer and then computes the Hmac over the raw payload, an oversized header can cause the application to copy more bytes than the buffer can hold. Even though Hanami encourages explicit, safe abstractions, developers who mix low-level buffer management with cryptographic verification risk introducing classic overflow patterns.

The interaction between Hmac Signatures and buffer overflow is particularly sensitive when the signature is computed over user-controlled data that determines buffer sizes. An attacker might send a carefully crafted message where the length field is inconsistent with the actual content, triggering an overflow during parsing before the Hmac is verified. This can lead to information disclosure or code execution depending on the runtime environment.

Because middleBrick scans unauthenticated attack surfaces and tests inputs such as headers and body fields in parallel, it can surface cases where oversized or malformed inputs related to Hmac workflows trigger unexpected behavior. Findings may map to OWASP API Top 10 categories such as Broken Object Level Authorization and Input Validation, with remediation guidance focused on strict size checks and avoiding manual buffer handling.

To mitigate buffer overflow risks while using Hmac Signatures in Hanami, ensure all inputs that influence buffer allocation are validated and bounded before use, and use language-provided safe abstractions for cryptographic operations.

Below are concrete, realistic code examples for a Hanami endpoint that verifies an Hmac signature safely.

# frozen_string_literal: true
require "openssl"
require "json"

class App::Controllers::SecureEndpoint
  include App::Action

  SECRET_KEY = ENV.fetch("HMAC_SECRET").freeze

  def call(params)
    # Extract the payload and signature from headers
    payload = request.body.read
    signature = request.headers["X-Hmac-Signature"]

    # Validate and bound inputs before processing
    unless valid_signature?(payload, signature)
      self.status = 401
      return { error: "invalid_signature" }.to_json
    end

    # Parse only after verification to avoid processing untrusted size hints
    parsed = JSON.parse(payload, symbolize_names: true)
    data = parsed[:data]

    # Enforce strict size limits on user-controlled fields
    if data.bytesize > 1024
      self.status = 400
      return { error: "payload_too_large" }.to_json
    end

    # Continue with business logic
    { status: "ok", data: data }.to_json
  end

  private

  def valid_signature?(payload, signature)
    return false if signature.nil? || signature.empty?
    expected = OpenSSL::HMAC.hexdigest("SHA256", SECRET_KEY, payload)
    # Use secure compare to avoid timing attacks
    ActiveSupport::SecurityUtils.secure_compare(expected, signature)
  rescue JSON::ParserError
    false
  end
end

In the above example, the body is read once and validated before any JSON parsing, avoiding reliance on untrusted size headers. The signature is computed over the raw payload, and a constant-time comparison prevents side-channel leaks. Input size is explicitly checked against a safe upper bound, preventing unbounded allocations that could lead to overflow in underlying C extensions.

For applications that accept numeric fields used for buffer sizing, always coerce and clamp values:

# frozen_string_literal: true
module Helpers
  def safe_buffer_size(raw_size)
    size = Integer(raw_size, exception: true)
    size = 0 if size < 0
    [size, 4096].min
  rescue ArgumentError, TypeError
    1024
  end
end

Using middleBrick’s CLI, you can scan endpoints that use Hmac Signatures to verify that no unchecked inputs reach buffer-sensitive code paths. The Pro plan enables continuous monitoring so that future changes to signature handling do not reintroduce unsafe patterns, and findings are mapped to compliance frameworks such as OWASP API Top 10 and SOC2.